This invention relates to seals for preventing leakage of a fluid about a shaft extending through a housing such as a pump. The invention is particularly concerned with a seal for containing high vapor-pressure liquids, such as liquefied light hydrocarbon gases and the method of sealing such fluids.
Prior to this invention, two spaced apart mechanical end face seals have been used for this application. This system was of the "wet," contacting type which had to be cooled by a buffer liquid pumped between the two spaced mechanical seals. The system for circulating the buffer liquid (frequently oil) includes pumps, coolers and reservoirs which add complexity and expense to the apparatus. Further, the mechanical seals, even with their complicated support system, often last only a few days in pumps handling liquefied light hydrocarbons.
It is known that a kind of mechanical end face seals, known as dry running gas seals, can be operated without oil lubrication when one of the faces is provided with a series of spiral grooves. Seals of this type are shown and described in U.S. Pat. No. 4,212,475, issued to Josef Sedy and assigned to the present assignee. The subject matter of this patent is incorporated by reference herein. This latter type of seal has been used to solve gas containment problems.
This invention seeks to provide an end face seal arrangement which is particularly appropriate for use in equipment handling liquids having high vapor-pressure such as liquefied light hydrocarbons. This is accomplished in part by ignoring, to some extent, the conventional wisdom which is to cool the opposing faces of the seal. Instead, the seal is intended to create shear heating of the sealed fluid between the seal faces of a first stage or module which, it is believed, causes phase change of the liquid so that a second stage dry running gas seal is effective to contain the vaporized fluid.